1. Field of the Invention
The present invention relates to a focusing optical system and, more specifically, to a focusing optical system for single-lens reflex cameras.
2. Description of the Prior Art
Illustrated in FIG. 1 is an example of the conventional focusing optical systems in which a detector element 2 is arranged on a focusing glass 1 so that a photographing lens of a single-lens reflex camera can be focused on the basis of the light intensity detected with the detector element 2. In another example of the conventional focusing optical systems, a semitransparent mirror is arranged in the focusing glass 1 and the light split by the semitransparent mirror is re-focused with a lens onto a detector element arranged, for example, beside the focusing lens for focusing the photographing lens of a single-lens reflex camera in the similar manner. In these conventional focusing optical systems, the central portion of the visual field in the view-finder is darkened or, in the worse cases, the image at such a portion is made quite invisible by the detector element on the focusing glass or the semitransparent mirror in the focusing glass, thereby resulting in adverse effect on observation through the view-finder.